[evlatests] Beam Cuts for ea21 -- notable issue with Horizontal Motion

[Todd Hunter]

Hi Rick,
     What was the azim/elev of this measurement?  I wonder if the target 
happened to be passing through its point of zero azimuth velocity, which 
can be challenging to track accurately (because the tachometer feedback 
goes to zero).  This situation only applies to sources that transit 
between the zenith and the pole. I worked this out awhile back for GBT 
servo tests.  Here is a (hopefully) self-explanatory diagram computed 
for the VLA's latitude that you can compare to:

https://safe.nrao.edu/wiki/pub/GB/PTCS/SiderealRateVelocities/azelrate.out.vla.sort.png

Todd

On 6/18/13 8:06 PM, Rick Perley wrote:
> We are attempting to characterize the behavior of the antenna motion 
> on ea21, prior to its move later this week to the barn for the ACU 
> retrofit.
>
>    To do this, we made four 'holographic' cuts through the main beam, 
> along the azimuth and elevation direction.  This was done at L and X 
> bands.    A 0.1 second integration was used (to search for rapid 
> oscillations), and 10 seconds per pointing.  Each cut is 11 points 
> long, and the oversampling is 5X -- essentially we're cutting through 
> the main lobe only.
>    Attached are two plots for X-band, for baseline 17 x 21.  Note that 
> for the vertical cut, the steps are crisp and rectangular, beyond ~1 
> second settling.  However, for the horizontal cut, something else is 
> going on, and even after ~10 seconds, there is no clear sense that the 
> antenna has stabilized.  This behavior is not unique to 21 -- others 
> show the same thing.  The same behavior is seen at L-band, although 
> the amplitude excursion is much smaller (as expected).
>    We plan to do 20-second pointings next (to see how long it takes to 
> really settled), and to do cuts at K-band, for which the deviations 
> should be much larger.
>
>
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